This is a study of clinical and immunopathologic events during the course of viral hepatitis in man. The objective is to elucidate the etiology and pathogenesis of the disease(s) and develop improved methods of prevention and management. In specific terms, this involves studies of the role of the hepatitis B surface antigen (HBsAg) and other hepatitis B virus (HBV) components in the pathogenesis of HBV infection, development of improved methods for detection of HBAg and other viral antigens, and investigation of the problem of so-called non-Type B post-transfusion hepatitis (PTH). Concomitantly, evidence for involvement of hepatitis A virus (HAV) and other putative hepatitis viruses will be sought in PTH and other forms of viral hepatitis.